Traditional UHP systems use a current-driven run-up scheme to heat up lamps after ignition. The current level is typically kept constant for a given time or until some conditions on lamp voltage are reached, after which the lamp is driven by a new constant current. A transition to the new current level can be smoothened by using a slow ramp and this process can be iterated a few times. In a typical run-up scheme the current level is increased stepwise several times until the target power level of the lamp is reached.
It is very difficult to design such schemes, i.e. the stepwise increase of the current, for the whole lamp voltage range, especially when lamp cooling is not in control by the lamp driver. This may lead to electrode tip damages due to high current peaks and determine a decrease of lamp performance, for example by decreasing lamp lifetime. It is important to notice that a lamp cooling has a big influence on lamp voltage both during transient and during steady state operation. In the majority of lamp systems currently available in the market the lamp driver has no control on the intensity of lamp cooling.
Among the requirements for running up a UHP lamp there are some limitations within which the lamp brightness must have reached a given percentage of its final value within a relatively short time. In order to achieve this requirement for a lamp which has a relatively low voltage value during its steady state operation, the level of currents used have to be significantly high and sometimes can exceed the maximum load for the lamp itself. This can temporarily damage the electrode tip and generate a brightness drop that, although recoverable, will be perceived and measured as a loss in performance. Moreover, the repeated operation at high current levels could permanently damage the lamp and reduce its lifetime. On the other hand, in case the lamp voltage during steady state is sufficiently high—which is the case after several hundreds or thousands hours of operation—driving it with relatively high currents during run-up can also lead to abnormal burn-back of the electrode tips. This in turn means reduced lifetime and reliability level.
WO 2006/072858 A2 discloses a lighting assembly and method of operating a discharge lamp, in which a method of running up the discharge lamp is described which is at least partly based on a power control. The lamp is operated in a first turn-on interval with increasing electrical power, but only up to an initial maximum power value less than the nominal power of the lamp. Then, during a power ramp interval, the lamp is operated with increasing electrical power over time. The electrical power increases from the initial maximum power value to nominal power. This power ramp interval is initiated at a time where the lamp has already reached initial stable operation conditions in order to achieve a reduction of electrode distance which is considered to limit electrode burn-back. This method of running up a discharge lamp, however, does not avoid current peaks.